1. Field of the Invention
The present invention relates to an electronic device and to a method of driving an electronic device. In particular, the present invention relates to an active matrix electronic device having a thin film transistor (TFT) formed on an insulating substrate, and to a method of driving an active matrix electronic device. From among all active matrix electronic devices, the present invention relates, in particular, to an active matrix electronic device using a self light emitting element, such as an OLED (Organic Light Emitting Diode) element, and to a method of driving such an active matrix electronic device.
2. Description of the Related Art
OLED displays have been gathering attention in recent years as flat display substitutes for LCDs (liquid crystal displays), and research into OLED displays is proceeding apace.
LCDs can roughly be divided into two types of driving methods. One is a passive matrix type using an LCD such as an STN-LCD, and the other is an active matrix type using an LCD such as a TFT-LCD. OLED displays are similarly divided roughly into two types; one a passive type, and the other an active type.
For a case of the passive type, wirings which become electrodes are arranged in portions above and below an OLED element. Voltages are applied in order to the wirings, and the OLED elements turn on due to the electric current flowing. On the other hand, each pixel has a transistor in a case of the active type, and a signal can be stored within each pixel.
A schematic diagram of an active type OLED display device is shown in FIG. 21A. A source signal line driver circuit 2151, a gate signal line driver circuit 2152, and a pixel portion 2153 are arranged on a substrate 2150. The gate signal line driver circuit is arranged on both sides of the pixel portion in FIG. 21A, but it may also be placed on only one side. A signal for driving the display device is input to each driver circuit in accordance with a flexible printed circuit (FPC) 2154.
FIG. 21B shows an enlargement of a portion of the pixel portion 2153, 3×3 pixels. The portion surrounded by a dotted line frame 2100 is one pixel. Reference numeral 2101 denotes a TFT which functions as a switching element when a signal is written into the pixel (hereafter referred to as a switching TFT). The switching TFTs may be n-channel TFTs or p-channel TFTs in FIGS. 21A and 21B. Reference numeral 2102 denotes a TFT (hereafter referred to as an OLED driver TFT) which functions as an element (electric current control element) for controlling the electric current supplied to an OLED element 2103. The OLED driver TFT is arranged between an anode of the OLED element 2103 and an electric current supply line 2107 when the OLED driver TFT is a p-channel TFT. As another type of separate structure, it is also possible to use an n-channel TFT or to arrange the OLED driver TFT between a cathode of the OLED element 2103 and a cathode wiring. However, a method in which the OLED driver TFT is arranged between an anode of the OLED element 2103 and the electric current supply line 2107 is best when using a p-channel TFT as the OLED driver TFT because the transistor operation is good with its source grounded and because of the constraints on the production of the OLED element 2103, and therefore this method is often employed. Reference numeral 2104 denotes a storage capacitor for storing a signal (voltage) input from a source signal line 2106. One of the terminals of the storage capacitor 2104 is connected to the electric current supply line 2107 in FIG. 21B, but it is also possible to use a dedicated wiring. A gate signal line 2105 is connected to a gate electrode of the switching TFT 2101, and the source signal line 2106 is connected to a source region. Further, the anode of the OLED element 2103 is connected to one of a source region and a drain region of the OLED driver TFT 2102, while the electric current supply line 2107 is connected to the remaining region.
Operation of the active type OLED element is explained. The relationship between the electric current flowing in an OLED element and the brightness of the OLED element is shown in FIG. 22A. It can be understood from FIG. 22A that the brightness of the OLED element increases nearly in direct proportion to the electric current flowing in the OLED element. The electric current flowing in the OLED element will therefore be mainly argued hereafter. Next, the voltage vs. Electric current characteristics of the OLED element are shown in FIGS. 22B and 22C. When a voltage exceeding a certain threshold value is applied to the OLED element, an exponentially large electric current begins to flow. From another point of view, even if the amount of electric current flowing in the OLED element changes, the value of the voltage applied to the OLED element does not change much. On the other hand, if the value of the voltage applied to the OLED element changes even by a small amount, the amount of electric current flowing in the OLED element changes greatly. It is therefore difficult to control the amount of electric current flowing in the OLED element, namely the brightness of the OLED element, by controlling the value of the voltage applied to the OLED element. The brightness in the OLED element is then controlled in accordance with controlling the amount of electric current flowing in the OLED element.
Refer to FIGS. 23A and 23B. FIG. 23A is a figure showing only the structure portions of the OLED driver TFT 2102 and the OLED element 2103 in the OLED element pixel portion of FIG. 21. An electric current supply line 2301, a cathode wiring 2302, an OLED driver TFT 2304, a gate electrode 2303 of the OLED driver TFT 2304, and an OLED element 2305 appear in FIG. 23A. FIG. 23B shows the voltage current characteristics in order to analyze the operational points of FIG. 23A. The voltage applied to the OLED element 2305 is taken as VOLED, the electric potential of the electric current supply line 2301 is taken as VDD, the electric potential of the cathode wiring 2302 is taken as VGND (=OV), the voltage between a source and a drain of the OLED driver TFT 2304 is taken as VDS, and the voltage between a gate electrode 2303 of to the OLED driver TFT 2304 and the electric current supply line 2301, namely the voltage between a gate and a source of the OLED driver TFT 2304, is taken as VGS. In order to clarify the explanation, it is assumed that a p-channel TFT is used as the OLED driver TFT 2304 here, and that a source terminal is set to the high side voltage terminal, while a drain terminal is set to the low side voltage terminal. As can be understood from FIG. 23B, the value of the electric current flowing in the OLED driver TFT 2304 becomes larger as the absolute value of the voltage between the gate and the source of the OLED driver TFT 2304 |VGS| gets larger.
Operational points of an OLED circuit are explained next. First, the OLED driver TFT 2304 and the OLED element 2305 are connected in series in the circuit of FIG. 23A. The value of the electric current flowing in both elements (the OLED driver TFT 2304 and the OLED element 2305) is therefore equal. The operation point of the circuit of FIG. 23A consequently becomes the point of intersection on the graph of the voltage current characteristics of both elements (see FIG. 23B.) VOLED becomes the voltage between VGND and the electric potential of the operation point in FIG. 23B. VDS becomes the voltage between VDD and the electric potential of the operation point. In other words, the voltage from VDD to VOLED is equal to the sum of VOLED and VDS.
A case in which VGS is changed is considered here. The OLED driver TFT 2304 is a p-channel TFT, and therefore becomes a conducting state if VGS becomes smaller than the threshold voltage Vth of the OLED driver TFT 2304. If VGS becomes even smaller, namely the absolute value |VGS| becomes additionally larger, then the amount of electric current flowing in the OLED driver TFT 2304 becomes additionally larger, and the value of the electric current flow in the OLED element 2305 naturally becomes larger as well. The brightness of the OLED element 2305 becomes higher in proportion to the value of electric current flowing in the OLED element 2305. However, VOLED also becomes larger at this point.
In order to analyze the operation in a rather detailed fashion, the operational region of the OLED driver TFT 2304 for a case in which |VGS| is large is discussed first. In general, the operation of a transistor can be roughly divided into two regions. One region is one in which the electric value of the electric current almost does not change even when there is a change in the voltage between the source and the drain; namely, a saturation region in which the current value is determined by only the voltage difference between the source and the drain (|VDS|>|VGS−Vth|). The other region is a linear one in which the value of the electric current is determined by the voltage between the source and the drain, and by the voltage between the gate and the source (|VDS|<|VGS−Vth|). The operation region of the OLED driver TFT 2304 is considered based upon the above. First, when the value of the electric current is low, namely in a case when |VGS| is small, the OLED driver TFT 2304 operates in the saturation region as shown in FIG. 23B. If |VGS| then becomes larger, the value of the electric current also becomes large. At the same time, VOLED also gradually becomes larger. Therefore, VDS becomes smaller the larger that VOLED becomes at this point. However, the OLED driver TFT 2304 is operating in the saturation region in this case, and even if VDS changes, the value of the electric current changes very little. In other words, when the OLED driver TFT 2304 is operating in the saturation region, the amount of electric current flowing in the OLED element 2305 is determined only by |VGS|.
In addition, if |VGS| becomes larger, the OLED driver TFT 2304 begins to operate in the linear region. Then VOLED gradually becomes larger. VDS consequently becomes smaller the larger VOLED becomes. In the linear region, the amount of electric current also becomes smaller if VDS decreases. Therefore, the value of electric current does not increase easily even if |VGS| becomes larger. Assuming the case that |VGS|=∞, the value of the electric current becomes equal to IMAX. Namely, however large |VGS| becomes, an electric current of more than IMAX will not flow. IMAX is the value of the electric current flowing in the OLED element 2305 when VOLED is (VDD−VGND)(VGND=0 V here, and therefore VOLED=VDD).
Bringing together the above operation analysis, when |VGS| is changed, the value of the electric current flowing in the OLED element is shown in a graph of FIG. 24. As the value of |VGS| becomes larger and exceeds the absolute value of the threshold voltage of the OLED driver TFT |Vth|, then the OLED driver TFT is placed in a conducting state, and electric current begins to flow. The value of |VGS| at this point is referred to as the turn on start voltage. If |VGS| becomes additionally large, the value of the electric current becomes larger, and finally the value of the electric current saturates. The value of |VGS| at this point is referred to as the brightness saturation voltage. As can be understood from FIG. 24, almost no current flows when |VGS| is smaller than the turn on start voltage. The amount of electric current changes in accordance with |VGS| when |VGS| is between the turn on start voltage and the brightness saturation voltage. When |VGS| then becomes sufficiently larger than the brightness saturation voltage, the value of the electric current flowing in the OLED element changes very little. Control of the value of the electric current flowing in the OLED element, namely control of the brightness of the OLED element, can thus be performed in accordance with changing |VGS|.
Operation of an active type OLED circuit is explained next. FIGS. 21A and 21B are again referred to.
First, the gate of the switching TFT 2101 opens when the gate signal line 2105 is selected, and the switching TFT 2101 is placed in a conducting state. The signal (voltage) of the source signal line 2106 is thus stored in the storage capacitor 2104. The voltage of the storage capacitor 2104 becomes the voltage VGS between the gate and the source of the OLED driver TFT 2102, and therefore the electric current, which responds to the voltage of the storage capacitor 2104, flows in the OLED driver TFT 2102 and in the OLED element 2103. As a result, the OLED element 2103 turns on. As explained by FIGS. 23A to 24, the brightness of the OLED element 2103, namely the amount of electric current flowing in the OLED element 2103, can be controlled by VGS. VGS is the voltage stored in the storage capacitor 2104, and is the signal (voltage) of the source signal line 2106. In other words, the brightness of the OLED element 2103 is controlled by controlling the signal (voltage) of the source signal line 2106. Finally, the gate signal line 2105 in unselected, the gate of the switching TFT 2101 closes, and the switching TFT 2101 is placed in a non-conducting state. The electric charge stored in the storage capacitor 2104 continues to be stored at this point. VGS is therefore stored as is, and the electric current in response to VGS continues to flow in the OLED driver TFT 2102 and in the OLED element 2103.
Information regarding the above explanation is reported in papers such as the following: “Current Status and Future of Light-emitting Polymer Display Driven by Poly-Si TFT”, SID99 Digest, p. 372; “High Resolution Light Emitting Polymer Display Driven by Low Temperature Polysilicon Thin Film Transistor with Integrated Driver”, ASIA DISPLAY 98, p. 217; and “3.8 Green OLED with Low Temperature Poly-Si TFT”, Euro Display 99 Late News, p. 27.
A method of gradation display of an OLED element is explained next. As FIG. 24 shows, when the absolute value of the gate voltage of the OLED driver TFT |VGS| is equal to or above the turn on start voltage and equal to or below the brightness saturation voltage, the brightness of the OLED element, namely the gray scale, can be controlled in an analog manner by changing the value of |VGS|. This method is therefore referred to as an analog gray scale method.
The analog gray scale method has a disadvantage in that it is weak with respect to dispersion in the electric current characteristics of the OLED driver TFTs. In other words, if the electric current characteristics of the OLED driver TFTs differ, the value of the electric current flowing in the OLED driver TFTs and the OLED elements will differ even if the same gate voltage is applied. As a result, the brightness of the OLED elements, namely their gray scale, changes. FIG. 25 shows a graph of the absolute value of the gate voltage of an OLED driver TFT |VGS| and the electric current flowing in the OLED element for a case in which the threshold voltage value and the mobility of the OLED driver TFT change. For example, the voltage effectively applied to the gate of the OLED driver TFT becomes smaller if the threshold voltage of the OLED driver TFT becomes larger (|VGS|−|Vth|), and therefore the turn on start voltage becomes larger. Further, if the mobility of the OLED driver TFT becomes smaller, then the electric current flowing between the source and the drain of the OLED driver TFT becomes smaller, and therefore the slope of the graph becomes smaller.
In order to reduce the effect of dispersion in the characteristics of the OLED driver TFTs, a method referred to as a digital gray scale method was proposed. This method is a method of controlling the gray scale by two states, a state in which the absolute value of the gate voltage of the OLED driver TFT |VGS| is below the turn on start voltage (when almost no electric current flows), and a state in which |VGS| is greater than the brightness saturation voltage (in which the value of the electric current is nearly IMAX). In this case, if the value of the absolute value of the gate voltage of the OLED driver TFT |VGS| is sufficiently higher than the brightness saturation voltage, the electric current value stays near IMAX even if the electric current characteristics of the OLED driver TFTs are dispersed. The influence of the OLED driver TFT dispersions can therefore be made extremely small. The gray scale is controlled by two states, an ON state (a bright state in which the maximum electric current flows) and an OFF state (a dark state in which the electric current does not flow), and therefore this method is referred to as the digital gray scale method.
However, only two gray scales can be displayed with the digital gray scale method in this state. Several techniques of changing to multiple gray scales by combining this method with another method have been proposed.
One of these techniques is a method in which a surface area gray scale method and a digital gray scale method are combined. The surface area gray scale method is a method of outputting gray scales by controlling the surface area of portions which are switched on. Namely, one pixel is divided into a plurality of sub-pixels, and the number of sub-pixels turned on and their surface area are controlled, and a gray scale is expressed. Disadvantages of this method include the fact that it is difficult to increase the resolution, and that it is difficult to make a lot of gray scales, because the number of sub-pixels cannot be made large. The surface area gray scale method is reported upon in papers such as: “TFT-LEPD with Image Uniformity by Area Ratio Gray Scale”, Euro Display 99 Late News, p. 71; and “Technology for Active Matrix Light Emitting Polymer Displays”, IEDM 99, p. 107.
Another method capable of making many gray scales is a method which combines a time gray scale method and a digital gray scale method. The time gray scale method is a method of outputting gray scales by controlling the amount of turned on time. In other words, one frame period is divided up into a plurality of subframe periods, and gray scales are expressed by controlling the number and the length of the subframe periods turned on.
A case of combining the digital gray scale method, the surface area gray scale method, and the time gray scale method is reported in “Low-Temperature Poly-Si TFT driven Light-Emitting-Polymer Displays and Digital Gray Scale for Uniformity”, IDW '99, p. 171.
A method applied for in Japanese Patent Application Laid-open No. Hei 11-176521 is discussed as a method of combining the digital gray scale method and the time gray scale method. A three bit gray scale is expressed here, and therefore as an example a case of dividing one frame period into three subframe periods is discussed.
FIG. 26 is referred to. As shown in FIG. 26, one frame period is divided into three subframe periods (SF). A first subframe period is referred to here as SF1. Subframe periods from the second onward are similarly referred to as SF2 and SF3. One subframe period is additionally divided into an address (write in) period (Ta) and a sustain (turn on) period (Ts). The sustain (turn on) period of SF1 is denoted by Ts1. The sustain periods for SF2 and SF3 are similarly denoted by Ts2 and Ts3.
Operations performed in the address (write in) period Ta are explained. FIGS. 21A and 21B, and FIG. 26 are referred to. First, the electric potential difference between the electric current supply line 2107 and a cathode wiring 2108 is set to 0 V. The electric potential of the cathode wiring 2108 is actually increased and placed at the same electric potential as that of the electric current supply line 2107. The cathode wiring 2108 is connected to all pixels, and therefore this operation is performed in all pixels simultaneously. The aim of this operation is so that no electric current flows in the OLED elements 2103, without depending upon the value of the voltage of the storage capacitor 2104 of each pixel. Signals (voltages) are then stored in the storage capacitors 2104 of each pixel through the source signal lines 2106. To set a pixel into a display state, the absolute value of the voltage between the gate and the source of the OLED driver TFT 2101 is set to a voltage sufficiently higher than the brightness saturation voltage. When a pixel is set to not display, the |VGS| of the OLED driver TFT 2101 is set to a voltage sufficiently lower than the turn on start voltage. The signals (voltages) are stored in the storage capacitors 2104 of all pixels. The operation of the address (write in) period Ta is thus complete.
The sustain (turn on) period Ts1 begins next. The electric potential difference between the electric current supply line 2107 and the cathode wiring 2108 was in a state of 0 V during the address (write in) period (Ta). In the sustain (turn on) period (Ts1), a voltage is applied between the electric current supply line 2107 and the cathode wiring 2108 simultaneously to all pixels. As a result, an electric current flows in the OLED driver TFT 2101 and in the OLED element 2103 of pixels in which |VGS| is sufficiently larger than the brightness saturation voltage, and the OLED elements turn on. An electric current does not flow in the OLED driver TFT 2101 and in the OLED element 2103 for pixels in which is sufficiently lower voltage than the turn on start voltage, and those pixels remain dark. This state continues, and the electric potential difference between the electric current supply line 2107 and the cathode wiring 2108 is once again set to a state of 0 V when the sustain (turn on) period Ts1 is complete. This naturally occurs across all the pixels simultaneously. Electric current then does not flow in the OLED elements 2103, without depending on the value of the storage capacitor 2104 voltage of each pixel, name |VGS|, and the OLED elements 2103 become dark.
The above is the operation of one subframe period (SF1). Similar operations are also performed in SF2 and SF3. However, the length of the sustain (turn on) periods differ in accordance with the subframe period. The length ratios become Ts1:Ts2:Ts3=22:21:20. In other words, the sustain (turn on) periods change in accordance with powers of 2. The changing of the sustain (turn on) period lengths by powers of 2 is in order to easily conform to digital operation.
The OLED element 2103 does not turn on during the interval until the end of the address (write in) period even if a predetermined voltage is applied to the gate of the OLED driver TFT 2101, and the OLED driver TFT 2101 is in a conducting state. The OLED element 2103 is made to turn on at the same time as the sustain (turn on) period begins. This is in order to more accurately control the length of the sustain (turn on) periods. A timing chart relating to the electric potential VGND of the cathode wiring of the OLED element 2103 is shown in FIG. 26. The cathode wiring is connected to all pixels, and therefore reference numeral 2601 denotes the electric potential VGND of the cathode wirings of all pixels in FIG. 26. The electric potential of the cathode wiring is set to the same electric potential as that of the electric current supply line, or to a higher electric potential, in the address (write in) period (Ta). the electric potential of the cathode wiring is then reduced in the sustain (turn on) period, and an electric current flows in the OLED elements.
The brightness is controlled by controlling whether or not the OLED elements turn on in the sustain (turn on) periods Ts1 to Ts3 in the gray scale display method. With this example, 23=8 turn on time lengths can be determined by combining the sustain (turn on) periods, and therefore 8 gray scales can be displayed. This method of performing gray scale display by thus utilizing the lengthening and shortening of the turn on times is referred to as the time gray scale method.
In addition, the number of divisions of one frame period may be increased for a higher number of gray scales. It becomes possible to express 2n gray scales, in which the ratio of lengths of the sustain (turn on) periods becomes Ts1:Ts2: . . . :Ts(n-1):Tsn=2(n-1):2(n-2): . . . :21:20 for a case of dividing one frame period into n subframe periods.
Note that gray scale display is also possible even when the lengths of the sustain (turn on) periods are not ratios of powers of 2.
The division of the subframe periods into address (write in) periods and sustain (turn on) periods, is in order to be able to freely set the length of the sustain (turn on) periods. In other words, it becomes possible to set the sustain (turn on) periods shorter than the address (write in) periods by dividing up the subframe periods. If the sustain (turn on) period is short for a case in which the period is not divided, then there are cases in which the address (write in) period overlaps with the address (write in) period of another subframe, and therefore normal signal write in is not performed.
Problems associated with the method of dividing into address (write in) periods and sustain (turn on) periods for a case of multiple gray scale in which the time gray scale method and the digital gray scale method are combined, namely the technique submitted in Japanese Patent Application Laid-open No. Hei 11-176521, is mainly discussed.
First, the fact that the OLED element is not turned on in the address (write in) period Ta can be given. The ratio of the display period to an entire one frame period (this is referred to as a duty ratio) therefore becomes small. Assuming that the ratio of the total time occupied by the sustain (turn on) periods (Ts) in one frame period is half, namely that the duty ratio is 50%, a brightness can be obtained which is only half that for a case in which the duty ratio is 100%. It is necessary that the brightness at the time light is emitted in the sustain (turn on) period, namely the instantaneous brightness, be twice as high in order to obtain a brightness equal to that of a case of a 100% duty ratio. It is therefore necessary for an electric current which is twice as large to flow in the OLED elements.
A second problem point is that it is necessary to complete the write in of the signals to all of the pixels within the address (write in) period (Ta), and therefore it is necessary to have high speed circuit operation. If the circuit operation is slow, then the address (write in) period (Ta) becomes longer. As a result, the duty ratio becomes smaller, and various problems develop. Further, the energy consumption becomes large if a high speed circuit operates, and this also becomes problematic.
A third problem point is that it is difficult to increase the number of pixels. The reason this is true is that the address (write in) period (Ta) becomes longer by increasing the number of pixels, and as a result, the duty ratio becomes smaller.
A fourth problem is that it is difficult to increase the number of gray scales. This is because it is necessary to increase the number of divisions in the subframe periods in order to increase the number of gray scales, and as a result, the number of address (write in) periods (Ta) increases, and the duty ratio becomes smaller.